1. Field of the Invention
The present invention relates to an optical system and an image pickup apparatus including the optical system, which are suitable for an image pickup optical system that is used for an image pickup apparatus such as a video camera, a digital still camera, a silver-salt film camera, or a broadcasting television camera.
2. Description of the Related Art
In an image pickup apparatus such as a single-lens reflex camera, as a method of changing a focal length of an image pickup optical system, a converter method in which a magnification conversion optical unit (an extender) is inserted into an optical path so as to change the focal length of a total system is known. For example, in an interchangeable lens for the single-lens reflex camera, a rear converter is mounted between the interchangeable lens and a lens mount of the camera body to be able to design a focal length of the interchangeable lens to be long, i.e. to obtain a long focus.
In a compact camera or the like, a front converter is mounted on an object side relative to a lens unit closest to the object side of an image pickup optical system, and thus the focal length of the total system can be elongated. In addition, a rear converter is mounted on an image side of a main lens system and thus the focal length of the total system can be changed to be long (Japanese Patent Laid-Open No. 2011-112725).
On the other hand, as a method in which a total lens length (a length from a first lens surface to an image plane) does not change before and after insertion of the converter, a method of inserting and removing the built-in magnification conversion optical unit in a relay unit of a zoom lens is known. In the zoom lens for a television camera, an afocal unit is provided at a relatively front side in the relay unit of the zoom lens, and the magnification conversion optical unit (the extender, hereinafter referred to as an EXT) is inserted into and removed from this afocal unit.
A zoom lens disclosed in Japanese Patent Laid-Open No. 2010-186179 is, in order from an object side to an image side, configured by a first lens unit that does not move in zooming, a second lens unit that monotonically moves to an image plane side in zooming, a third lens unit that moves to correct a variation of the image plane caused by zooming, and a fourth lens unit that does not move in zooming. In this configuration, the extender is inserted and removed at a position where an air gap is the widest in the fourth lens unit so as to change a range of the focal length of the total system.
As a zoom lens that has an image stabilizing function to correct a blur of an image caused by a vibration of the zoom lens, a zoom lens that shifts a part of a lens unit constituting the zoom lens in a direction perpendicular to an optical axis so as to correct the blur of a shot image is known.
A zoom lens disclosed in Japanese Patent Laid-Open No. 2001-281546 is configured by a first lens unit that does not move in zooming, a second lens unit that is a zoom unit (a magnification varying unit), a third lens unit that corrects an image plane varying in accordance with zooming, and a fourth lens unit that does not move in zooming. Furthermore, a lens unit for image stabilization is contained at a position closest to the object side in the fourth lens unit.
The converter method in which the magnification conversion optical unit is built in apart of the optical system can change the range of the focal length of the total system to a telephoto side without changing the total lens length by inserting the magnification conversion optical unit. However, when the magnification conversion optical unit is inserted and removed, in order to maintain an optical performance appropriately, it is necessary to appropriately set a lens configuration of the magnification conversion optical unit and also to insert and remove the magnification conversion optical unit at an appropriate position in an optical path.
For example, if configurations of optical systems that are located ahead and behind the magnification conversion optical unit are inappropriate, the optical performance is significantly reduced when the magnification conversion optical unit is inserted. Furthermore, a size of the magnification conversion optical unit is increased, and therefore rapid insertion and removal are difficult.